JavaVMHelper keeps the vm instance supplied with JNI_OnLoad and
it is reused whenever JniEnv is requested.
Plus, AndroidRuntime::registerNativeMethods() is replaced with
jniRegisterNativeMethods() in libnativehelper.
Test: pass MediaPlayer2Test#testPlayVideo
Bug: 63934228
Change-Id: Ia773afa93df00b32debd08f0a66893656f710341
Forked from 9070e295864b83deca9020ca8bdd9ff790e631a9
Test: plays local files, callback source and streaming.
Bug: 63934228
Change-Id: I692007c07eb2819f8ad1d843e4e2b1fc96fbecca